Effect of pore sizes on the microstructure and properties of the biomedical porous NiTi alloys prepared by microwave sintering

Biomedical porous NiTi alloys with tailored pore sizes at the same porosity levels (∼50%) were prepared by microwave sintering technique and space holder method. The effects of pore sizes on the microstructure, macrohardness, compressive strength, elastic modulus, superelasticity and corrosion resistance of the porous NiTi alloys were investigated. The results showed that the porosities of the porous NiTi alloys mainly depended on the weight ratio of the NH4HCO3 space holder agents, while the pore sizes mainly depended on the particle sizes of the space holder agents. The porous NiTi alloys mainly consisted of the predominant NiTi phase and the secondary phases Ti2Ni and Ni3Ti. With increasing the pore sizes of the porous NiTi alloys, the Ti2Ni phase and Ni3Ti phase increased, while the NiTi phase decreased. The hardness, compressive strength and elastic modulus of the porous NiTi alloys presented “S” type variation tendency with the pore sizes, respectively. The superelasticity of the porous NiTi alloys could reach ∼4% under the pre-strain of 5% as the pore sizes smaller than 294 μm. Moreover, partial shape memory effect was observed for all pore size levels under the experiment conditions. The increase of the pore sizes had adverse effects on the corrosion resistance of the porous NiTi alloys in Hank's solution.